This invention relates to an adjustable boom apparatus for supporting equipment, and in particular a spring biased self-balancing boom apparatus which compensates for spring force variation during adjustment of the boom.
Booms are used for mounting equipment to extend the useful range of the equipment over a greater area and to increase maneuverability and operator convenience in using the equipment. Conventionally, booms are counterbalanced by a mechanism which includes a spring attached to the proximal end of the boom, behind its pivot point. The counterbalancing mechanism counteracts the gravitational force on the boom and the equipment carried on the end of the boom. The spring of the counterbalancing mechanism is caused to retract or extend as the distal end of the boom is raised or lowered during boom adjustment. Most known counterbalancing arrangements rely on friction braking or other such means to maintain desired positions of boom adjustment. Such arrangements are characterized by jerky or otherwise uneven arm adjustments during positioning of the equipment.
One spring counterbalancing arrangement for an equipment support boom, depicted in U.S. Pat. No. 4,277,044, alleviates these problems by providing automatic compensation for inherent variations of spring extension forces which occur during adjustment of the boom. In this arrangement, the spring has one of its ends connected to the base which supports the boom and its other end attached to the boom by a mechanism which is adapted to alter the arm length-to-moment ratio and spring force angle according to variations in spring extension forces. To this end, the mechanism includes a first cam and a first follower constructed and arranged to effect extension of the spring when the distal end of the boom is lowered and vice versa, and a second cam and follower constructed and arranged to simultaneously move the opposite end of the spring toward the location of proximal pivoting of the arm when the distal end of the arm is lowered and vice versa.
In this arrangement, achieving compensation for spring extension forces requires a cam of a particular configuration providing a non-linear cam surface to effect requisite repositioning of one end of the spring relative to the pivot axis for the boom as the position of the boom is adjusted. Because the cam directs the repositioning of the end of the spring during adjustment of the boom, the degree of uniformity in compensation over the entire excursion of the boom is dependent upon manufacturing tolerances of the non-linear cam.
Moreover, this arrangement requires that the spring be pivoted about an axis as it is being extended or allowed to retract. Such spring movement increases wear and tear on the spring, and the spring could eventually break. The spring is attached to the boom at the back end thereof, behind the pivot point, and is normally maintained in tension. Thus, if the spring should break, the boom will drop, damaging the equipment it is carrying and possibly causing injury to a person working beneath the boom. Although this problem could be alleviated by using several springs, breakage of one of the springs could result in increased forces on the other springs, causing a chain reaction with other springs breaking and the boom dropping.